This invention relates generally to unified devices for rectifying alternating current, and more particularly to a highly compact rectifier unit which incorporates a heat sink and has a significantly higher current rating and a lower thermal impedance than similar rectifier units.
A conventional silicon rectifying diode includes a single pn junction, the diode characteristic being such that current can freely flow in only one direction, flow in the opposite direction being blocked by the junction. The ratings of silicon diodes include voltage, current and junction temperature values. Proper circuit design must ensure that the average junction temperature will never exceed its design limit; but good design practice, which takes into account the reliability factor, limits the maximum junction temperature to a lower value. Silicon rectifier ratings are normally specified at a given case temperature and operating frequency.
An avalanche diode differs from a conventional silicon diode in its ability to absorb large reverse energy pulses of a joule or more. This capability results from the fact that avalanche breakdown is a bulk crystal phenomenon, whereas edge breakdown of a conventional diode occurs because of defects in the crystal lattice. While there are differences in reverse characteristics, the ratings of current, forward voltage drop, reverse blocking voltage and operating temperatures for an avalanche diode are similar to those of a conventional diode of the same wafer junction size.
The concern of the present invention is with rectifier units constituted by a set of silicon diodes, preferably of the avalanche type, connected in a classic rectifier circuit arrangement. It is known to house a rectifier circuit of this type in a small case, the diodes and the strap interconnections therebetween forming a sub-assembly which is encapsulated in the case by a suitable potting compound that serves to protect the diodes from contamination and adverse environmental factors such as humidity. Only the rectifier circuit input and output terminals project from the case.
A compact rectifier unit of this known type serves to replace larger stud assemblies and offers an appreciable cost and size reduction for power supplies, converters, inverters, motor controls, DC motor starters and in other applications requiring rectification.
In operation, a rectifier generates a considerable amount of heat, particularly under heavy load conditions. Unless this heat is quickly dissipated, the rectifier will be incapable of carrying a large amount of current without doing damage to its diodes. Because the epoxy potting compound usually used in an encapsulated rectifier circuit is a relatively poor heat conductor, and the diodes are in close proximity to each other within a confined case, these conditions impose distinct limits on the safe current ratings of such units.
Though the invention will be described in conjunction with a single-phase full-wave bridge rectifier, it is applicable to a family of rectifier circuits, such as single-phase doublers employing a pair of diodes in series, three-phase half-wave rectifiers making use of three diodes and three phase full-wave rectifiers constituted by six diodes, as well as various forms of hybrid rectifier circuits which include thyristors in combination with diodes, all of which arrangements make use of conductive straps to effect the necessary interconnections of the rectifier circuit.
In rectifier circuit design, one must give careful consideraation to the dissipation of heat. Semiconductor devices with power ratings greater than one watt are usually provided by the manufacturer with a large flat surface which is intended to be clamped against a metal heat exchanger. The purpose of the heat exchanger is to transfer heat generated in the semiconductor device to the sink and then to the ultimate cooling medium, usually the surrounding air, without allowing the internal temperature to exceed the manufacturer's maximum specified temperature limits. Although the term "sink" in its most accurate sense refers to the ultimate cooling medium, in engineering terminology, it is loosely used to mean the heat exchanger.
When power is dissipated in a diode junction, heat must be conducted from the junction through intervening layers of material to the heat sink, which layers are said to possess thermal impedance.
The flow of heat through a thermal impedance gives rise to a temperature difference which is proportional to the rate of heat flow. Thermal impedance obeys, as it were, a thermal ohms law, thermal impedance being euivalent to electrical impedance, the temperature being comparable to voltage and the power being comparable to current. Thermal impedance is equal to .theta..sub.JC where .theta. is degrees C per watt, J is the junction temperature, and C is the case temperature. Obviously, the lower the thermal impedance, the greater the heat dissipation.
The value of thermal impedance is a function of the thermal design of the device itself. The thermal conductivity and dimensions of the various parts of the assembly and of the solders which bond the parts together determine how much heat can be transferred from the junction to the case mounting surface while maintaining the junction temperature within the permissible maximum limit. Although a user of a rectifier device has no control over this parameter, to properly apply the device he must know its value.
The problem of heat dissipation in encapsulated or integrated circuit components has been addressed in several prior patents. Thus the Murari et al. U.S. Pat. No. 3,911,327 discloses an integrated circuit chip potted within a casing having a thermal mass embedded in the package to act as a heat sink therefor. The Flaherty U.S. Pat. No. 3,157,828 shows an encapsulated printed circuit with heat transfer means including a heat sink. The use of heat sinks in conjunction with rectifiers is also shown in the Zellmer U.S. Pat. No. 3,668,477 and the Schuler U.S. Pat. No. 3,229,188.
But where the heat-generating device is in the form of a circuit package whose casing is adapted to accommodate the rectifiers and connectors, as in the Bernstein U.S. Pat. No. 3,307,077, it has not heretofore been known to incorporate a heat sink therein. Because of space limitations, one cannot embed a heat sink in the potting compound. Moreover, even if this were possible, it would be necessary to have the heat sink in some way exposed to the atmosphere for proper heat dissipation, and this cannot be done with an embedded heat sink.
On the other hand, should the base of the case be made in the form of a metal plate functioning as a heat sink, it would be necessary to electrically isolate the base plate from the rectifier sub-assembly, and this insulation would ordinarily function as a thermal barrier between the sub-assembly and the heat sink so that the thermal impedance would inevitably be high.